Such UO.sub.2 sintered pellets are tightly enclosed in coating tubes made of zircaloy, and are used as nuclear fuel.
Recently, in order to make the life of the nuclear fuel to be long to enable continuous operation for a long period of light water reactors or fast breeder reactors, it is advanced to realize a high degree of combustion of nuclear fuel. When the nuclear fuel is allowed to have a high degree of combustion, the amount of fission products (FP) generated from nuclear fuel pellets is increased. Among the products, gaseous one such as radon (Rn) scarcely make solid solutions in the matrix of the nuclear fuel pellet, which diffuse into the crystal grain boundary and generate bubbles there. Swelling occurs due to the bubble formation, and the volume of the pellet increases to give stress to the coating tube. This makes a cause to generate a mechanical interaction (PCI, Pellet Clad Interaction) between the pellet and the coating tube. In addition, the FP gas diffused into the grain boundary is released to the exterior of the pellet later, which increases the internal pressure of the fuel rod to make a cause to decrease the thermal conductivity of the gap between the pellet and the; coating tube.
In order to prevent the increase in PCI and the decrease in the thermal conductivity, it has been attempted that the nuclear fuel pellet is allowed to have a large grain size so as to enclose the FP gas in the pellet. This is based on the fact that although the generation of the FP gas itself cannot be suppressed, when the pellet is allowed to have a large grain size, for example, when the crystal grain size is made to be two-fold, the arriving distance to the grain boundary of the FP gas generated in the crystal grain becomes two-fold, and consequently the release speed of the FP gas becomes half.
Until now, methods for increasing the crystal grain size of the UO.sub.2 sintered pellet have been disclosed in Unexamined Published Japanese Patent Application No. 2-242195/1990; Unexamined Published Japanese Patent Application No. 3-287096/1991; Unexamined Published Japanese Patent Application No. 4-70594/1992 and the like.
According to these methods, nuclear fuel pellets having crystals of a large grain size of 20-120 .mu.m are obtained.
In the prior art, not only for the nuclear fuel pellets produced by these methods as a matter of course, but also for nuclear fuel pellets produced by other methods, the crystal grain size has been mainly measured by a cross-sectional method defined in accordance with ASTM E-112.
In this cross-sectional method, at first a produced UO.sub.2 sintered pellet is embedded in a synthetic resin, the pellet embedded in the resin is cut, and then its cross section is polished. Next, a wet etching treatment is performed to expose crystal grain boundaries of the pellet, and then the crystal grain boundaries are photographed by an optical microscope or the like. Next, in a state in which a scale line having a predetermined length is projected on a screen using a slide type projector, a photographed negative film is projected on the same screen so as to overlay a grain boundary texture to the scale line. The number of grains intersecting the scale line on the screen is measured at a plurality of places by sliding the negative film, and an average value of crystal grain sizes is determined according to the grain number.
However, in the above-mentioned cross-sectional method, there is such an advantage that the crystal grain size of the UO.sub.2 sintered pellet is directly observed by the photographing with the optical microscope or the like, and a value relatively having a high accuracy is obtained, but on the contrary, it is necessary that every time when the measurement is performed, the UO.sub.2 powder is placed in a mold frame to conduct formation and calcination to make the sintered pellet, as well as complicated works for preparation of the measurement are required, and fine operation and observation must be performed.
For example, when UO.sub.2 sintered pellets having large crystal grain sizes are produced for a high degree of combustion of nuclear fuel, in order to decrease the ratio of deficiency of the sintered pellets, it is necessary to perform judgment of the suitability of raw material powders for the sintered pellets beforehand. However, in the case of the conventional measurement method, there has been such a problem that relatively much time is consumed for this judgment, and consequently management cost for the raw material powder is raised.
An object of the present invention is to provide a method for estimating crystal grain sizes of UO.sub.2 sintered pellets without actually producing UO.sub.2 sintered pellets.
Another object of the present invention is to provide a method for estimating crystal grain sizes of nuclear fuel pellets in which when UO.sub.2 sintered pellets having large crystal grain sizes are produced for realizing a high degree of combustion of nuclear fuel, the judgment of suitability of raw material powders for the sintered pellets can be performed rapidly and economically.